1. Field of the Invention
This invention relates to a power converting apparatus and a power converting method for driving a medium to high voltage AC motor at a variable speed, and particularly to a power converting apparatus and a power converting method of a pulse width modulation (PWM) controlling system.
2. Discussion of the Background
Conventionally, for variable speed driving of a high voltage AC motor, a system which employs a high voltage invertor or another system wherein a step-down transformer and a step-up transformer are connected to the input side and the output side of a low voltage invertor to drive the high voltage AC motor is employed.
FIG. 6 is a circuit diagram of a driving circuit which employs a high voltage invertor of a conventional example, and FIG. 7 is a concept diagram illustrating four quadrature operation based on the relationship between the torque and the speed of a motor. In FIG. 6, reference symbol 10 denotes a high voltage AC motor of an object of driving, 101 an invertor unit, 102 a smoothing capacitor unit, 103 a regenerative converter unit, 104A and 104B each denotes an AC reactor, and 105 denotes a three-phase transformer.
The invertor unit 101 includes three-level invertors of the neutral clamping type and employs, for power elements, a GTO (Gate Turn Off Thyristor, hereinafter referred to simply as GTO) to assure a high withstanding voltage for the elements. The power elements are connected in series to achieve divisional sharing of a voltage, and variable voltage variable frequency (VVVF) power is supplied from a high voltage DC power supply formed from the smoothing capacitor unit 102 to the invertor unit 101. In order to keep divisional sharing of a voltage of the GTOS, well known snubber circuits must be installed individually. In the converter unit which supplies a DC voltage to the smoothing capacitor unit 102, the capacity of the high voltage invertors is generally as high as several hundreds kW or more, and the construction of the regenerative converter unit 103 is used for damping energy processing upon deceleration or for four quadrature operation (forward driving, reverse driving, forward regeneration and reverse regeneration) illustrated in FIG. 7. In FIG. 6, two circuits each composed of a combination of thyristors and GTOs are used in series connection, and control between driving and regeneration is performed depending upon the direction of DC power. The regenerative converter unit 103 is connected to secondary windings of the three-phase transformer 105 through the AC reactors 104A and 104B while primary windings of the three-phase transformer 105 are connected to a high voltage commercial power supply so as to receive supply of power.
FIG. 8 is a circuit diagram showing a driving circuit which employs a low voltage invertor of a conventional example. In FIG. 8, reference numeral 10 denotes a high voltage AC motor of an object of driving, 106 an invertor unit, 107 a smoothing capacitor unit, 108 a regenerative converter unit, 109 an AC reactor, 110 a step-down transformer, and 111 a step-up transformer.
The invertor unit 106 includes IGBTs (Insulated Gate Bipolar Transistors, hereinafter referred to simply as IGBTs) and diodes connected in a three-phase bridge circuit and is pulse width modulation (hereinafter referred to simply as PWM) controlled so that it may output a voltage and a frequency necessary to drive the motor 10 through the step-up transformer 111. Since the invertor unit 106 is a low voltage invertor, it is connected to the high voltage AC motor 10 through the step-up transformer 111. Also the regenerative converter unit 108 is composed of IGBTs and diodes connected in a three-phase bridge circuit similarly as in the invertor unit 106, and is connected to secondary windings of the step-down transformer 110 through the AC reactor 109 while primary windings of the step-down transformer 110 are connected to a high voltage commercial power supply so as to receive supply of power. Meanwhile, also DC buses of the regenerative converter unit 108 and the invertor unit 106 are connected to each other through the smoothing capacitor unit 107. Both of the invertor unit 106 and the regenerative converter unit 108 are PWM controlled.
As other motor driving systems, for example, a multiple cycloconverter recited in "Cycloconverter Apparatus" disclosed in Japanese Patent Laid-Open Application No. Heisei 6-245511 and a PWM cycloconverter recited in "Power Converting Apparatus of a Pulse Width Controlling System" disclosed in Japanese Patent Publication Application No. Heisel 7-44834 are known. However, they are not directed to driving of a high voltage AC motor described above.
Meanwhile, the trend of the world is directed to energy conservation, resource conservation, minimum size, high efficiency and low-distortion voltage and current waveform for improvement in environment, and due to complication of application systems, improvement in operation reliability such as improvement in regard to redundancy is required. Also the motor driving systems of the prior art described above naturally become an object of such improvement.
However, from the point of view of energy conservation, resource conservation, minimum size, high efficiency and low-distortion voltage and current waveform for improvement in environment, both of the high voltage invertor system and the low voltage inverter system of the prior art examples described above have the following problems.
In the case of the high voltage invertor system of FIG. 6, a GTO is employed for main circuit elements in order to achieve a high voltage withstanding property. Since a GTO is not a high speed switching element, it is difficult to use a high carrier frequency, and reduction in noise in inverter driving or suppression of waveform distortion cannot be anticipated. Further, since a snubber circuit of a GTO repeats charging and discharging each time switching is performed, also the loss is high, and since it has a circuit construction which employs a high voltage element, assurance of insulation for a main circuit, a bus bar and so forth is required and the snubber circuit is not suitable for minimizing the inverter package. Furthermore, since a GTO driving power supply is required for each GTO and besides a high voltage is applied between control power supplies, it is not easy to generate the control power supplies, and this is a bottle neck to minimize the inverter package.
Meanwhile, in the case of the step-up system of the low voltage inverter of FIG. 8, since it is an IGBT invertor of a low voltage, while high fr equency PWM control is possible and reduction in noise can be anticipated, in order to achieve a large capacity, parallel connection of IGBTs is required, and a countermeasure for parallel balancing and a snubber circuit are required and minimum size is difficult. Further, also an increase in loss of IGBTs, bus bars and snubber circuits arising from high current is estimated, and minimization is difficult also from the phase of cooling. Furthermore, where step-up is performed by a transformer as seen in FIG. 8, since the switching speed of IGBTs is high, that is, since dV/dt upon switching is large, also there is another drawback that, by inductances of wiring lines, floating capacitances of the wiring lines, inductances of a transformer and so forth, a resonance voltage is generated in synchronism with switching of PWM control of an invertor, causing dielectric breakdown of the motor. As a countermeasure against the drawback, it has been proposed to insert a filter between the invertor unit 106 and the step-up transformer 111 of FIG. 8 as recited in "Output Filter Circuit of Voltage Type PWM Invertor" disclosed in Japanese Patent Laid-Open Application No. Heisei 1-72144. In addition, since, upon low frequency operation, the voltage/frequency ratio to be provided to the transformer is set to 1.5 to 2 times that in the proximity of a rated frequency by an invertor in order to assure starting torque, there is another problem that a larger transformer than a transformer for a commercial frequency is required so that magnetic saturation may not occur. Further, if the invertor 106 generates an offset voltage due to a dispersion in switching characteristic of the IGBTs and so forth, then since a DC voltage is applied to the step-up transformer 111, magnetic saturation occurs. Consequently, there is a problem also that excessive current flows.
As a countermeasure against harmonic distortion of an output voltage or current, while the high voltage invertor is controlled by 3-level control and both of PWM control and amplitude control are used, the low voltage invertor system employs only PWM control and exhibits large harmonic distortion. Also for the power supply voltage, since the regenerative converter unit 103 of the high voltage invertor system of FIG. 6 uses 120-degree energization waveforms, low order harmonic distortion remains, and with the low voltage invertor system of FIG. 8, since the regenerative converter unit 108 performs PWM control, although low order harmonics of power supply current are suppressed, high order harmonics remain.
As described above, the conventional invertor systems cannot solve the technical subjects such as energy conservation, resource conservation, minimum size, high efficiency and low-distortion voltage and current waveform for improvement in environment needed by the market. Further, any of the systems cannot solve the technical subject of improvement in redundancy such that, upon failure, operation is performed with a normal part.
Further, of the systems other than the invertor systems, the cycloconverter recited in "Cycloconverter Apparatus" disclosed in Japanese Patent Laid-Open Application No. Heisei 6-245511 cannot raise, since it is of the power supply commutation system, the output frequency only up to 1/3 to 1/2 the power supply frequency. Consequently, the cycloconverter is not suitable for motor driving.
An improvement of the cycloconverter just described is a PWM cycloconverter recited in "Power Converting Apparatus of the Pulse Width Controlling System" disclosed in Japanese Patent Laid-Open Application No. Heisei 7-44834. The PWM cycloconverter has the following characteristics.
1) Miniaturization is easy because it does not require such a DC circuit as is required by an invertor system.
2) The element loss is low and the efficiency is high because the number of elements inserted in series in a route from a power supply to a load is smaller than that of an invertor system.
3) Four quadrature operation is easy because direct AC-AC conversion is used.
However, since also this system is a PWM control power converting system of three-phase inputs and three-phase outputs, although low order harmonics of power supply current are suppressed, high order harmonics remain, and the technical subject of voltage and current waveform distortion suppression cannot be solved for both of the input and the output. Further, in order to drive a high voltage AC motor, a system wherein a power element is so formed as to have a high voltage withstanding property to make a high voltage PWM cycloconverter or a voltage is raised by a transformer is adopted, and the same subjects as those of the high voltage inverter system or the transformer step-up system of a low voltage invertor occur. Furthermore, in the conventional examples described above, all of the systems have a problem that, if some function is damaged, then operation cannot be continued.